Ablative radiotherapy is a very effective way to non-invasively destroy a tumor. Tumors in the body, however, move with respiration and require precise targeting for this high dose treatment. There are a variety of technologies that implement tumor tracking to achieve ablative radiotherapy in the body; however, these are limited to treat just a few lesions in a patient due to the complexity and invasiveness of the necessary motion management techniques. Therefore, patients that present with later stage disease with more than a few metastatic sites (e.g. 5-10 lesions) are not treatable with ablative radiotherapy. RefleXion Medical is developing the first universal method for direct real-time targeting of tumors based on biological guidance. Utilizing positron emission tomography (PET) emissions from the patient, this new technique allows for continuous and automatic dose conformation to multiple lesions, enabling dose escalation to the target(s) and toxicity reduction to sensitive tissues. The concept has been proven with phantom experiments and monte-carlo software simulations. This proposal has two specific aims to mitigate the key risks in the development of the first clinical prototype hardware system. Aim 1 will evaluate the effects that the radiation source or linear accelerator (linac) system may have on the PET system on a bench-top, and vice-versa. Aim 2 will demonstrate simultaneous operation of the linac and PET systems on a closed-ring gantry. Success of these aims will enable the full clinical prototype to be completed and subsequent commercialization of the first biologically guided radiotherapy system.